This package is a copy of the JPA API classes from the Apache Geronimo Project. https://svn.apache.org/repos/asf/geronimo/specs/trunk/geronimo-jpa_3.0_spec/src/main/java/javax/persistence/
It is usefull to reuse JPA entity classes from your domain model into GWT client-side application. You can import the javax.persistence emulated classes by adding the following import to your .gwt.xml file:
<inherits name="com.googlecode.gwtx.javax.Persistence"/>